The present invention relates to a digital camera having an imaging element such as a CCD (charge-coupled device) and a MOS (metal oxide semiconductor), and particularly to an SLR (single-lens reflex) digital camera having a shutter.
An imaging element used in a digital camera for capturing an object image such as a CCD and a MOS generally includes a multiplicity of photo receiving cells (i.e., pixels) that are arranged uniformly in matrix. Due to the uniformity of the pixels, when an object image with a pattern is captured by the imaging element, an interference occurs between the two patterns, and a false color (i.e., moiré fringe) is viewed. To avoid such a false color from occurring, an optical low pass filter (hereinafter referred to as LPF) is provided to an image receiving area of the imaging element so as light with specific higher spatial frequency to cause interferences is blurred. An example of such technique is disclosed in Japanese Patent Provisional Publication No. P2004-173139. For such an optical LPF, various types of materials are used, including a birefringent crystal panel and a lithium niobate panel. Also, a diffraction grating, which splits light with a specific wavelength, and a phase filter, which cuts off light with a specific wavelength, is used.
When an imaging element with such an optical LPF is applied to a digital camera with a shutter, such as an SLR, the optical LPF is arranged between the shutter and the imaging element. As seen in an SLR camera illustrated in FIG. 7, for example, on a front surface of the camera body, a lens mount is provided, to which a photographing lens unit (not shown) is detachably attached. Light passed through the photographing lens is reflected by a movable half mirror (quick return mirror) 11 and forms an image on a focusing glass 12. The image formed on the focusing glass 12 is further reflected inside a pentaprism 13 and can be observed through an eyepiece lens 14 Behind the movable half mirror 11 is provided a focal-plane shutter unit 18. Further behind the shutter unit 18 is provided an imaging element 20 which captures an object image when a shutter of the focal-plane shutter unit 18 is opened. Furthermore, an optical LPF 19 is provided in front of the imaging element. It should be noted that, in a camera, an infra-red cutoff filter is generally provided to the imaging element in addition to the optical LPF 19, however, the infra-red cutoff filter in this example is integrated to a protective glass that is provided to the surface of the imaging element 20.
It should be noted that such an optical LPF (e.g., the birefringent crystal panel or the diffraction grating) requires a considerable thickness, therefore an interspace between the shutter unit 18 and the imaging element 20 may be large. In that case, the imaging element 20 is required to be arranged in a farther position from the shutter unit 18. For this reason, the camera should be configured to be large in the direction of the optical axis.
In addition, it should be noted that, when dirt and/or dust adhered to the surface of the optical LPF is removed by a user using a cleaning tool, the shutter is maintained open. However, if a battery is exhausted or an AC (alternating current) adapter is unexpectedly removed from the camera during the cleaning, or a finger of the user pressing a shutter button to hold the shutter open by a bulb setting is unintentionally removed, the shutter closes and may be damaged by the cleaning tool.